The present invention relates generally to a temperature measurement method and means, and more specifically to a non-invasive method for measuring the surface temperature of growing crystals.
Temperature is frequently an important parameter systems where materials are processed or their properties are to be measured. Many methods have been developed for measuring the temperature of solid or fluid phases of materials at both high and low temperatures. These methods may be classified as contact or non-contact methods. The contact methods (referred to as "invasive" methods) usually involve placing a sensor within or very close to the object to be measured, and then monitoring a temperature dependent property of the sensor, such as an electrical, optical, or geometrical property, or the like. It is generally essential for accurate measurements that the sensor disturbs the object as little as possible.
The non-contact ("non-invasive") methods of temperature measurement use a sensor which is remote from the object, and they measure a temperature dependent property of the object itself, such as the thermal (Planck) blackbody radiance, some feature of the optical spectrum, or the like. Methods which utilize the impingement of a probing beam onto an object to be measured in order to produce a measurable reflectance, absorption, or fluorescences response are, strictly speaking, invasive. However, particularly when photon beams are used as a probing beam, it is often possible to minimize the disturbance of the object by using a beam of very low intensity. The use of such a low intensity probe beam may be considered to be effectively nonintrusive.
In some situations, such as the growth of single crystals in various media, it is of interest to measure the temperature of the surface, as distinct from the bulk of a material. In several methods of crystal growth, a temperature gradient is established in a fluid medium to transport material or heat to or from a crystal which is growing in that medium. The thermal gradient normally extends into the crystal, so the surface and subsurface ("bulk") regions are at different temperatures. Since the quality of the crystal depends on controlling the temperatures of the crystal faces as they grow, it is the surface temperature which is of particular importance.
Many types of sensors are available for measuring the temperatures of solid or fluid phases in most situations, except that contact senors are not available for very high temperature measurement. However, contact sensors are usually inappropriate for measuring surface temperature because of their intrusive nature. The process of growing a single crystal of controlled composition and low defect content requires that precise control be maintained over the local thermal and composition fields at the surface of the crystal. A small disturbance of these fields can greatly affect the growth process. The presence of a sensor, however unobtrusive, can introduce disturbances such as contamination, mechanical damage to the growing crystal face, or alteration of the local temperature due to the thermal mass and conductivity of the sensor. For these reasons, non-invasive methods or photon probe methods with negligible impact are preferable in such cases, when they are available.
Single crystals of the tetragonal form of mercuric iodide (".alpha.-HgI.sub.2 ") have several important applications, and the growth of such crystals are of particular interest to the present inventors. Particular problems are encountered in the growth of .alpha.-HgI.sub.2 crystals due to the softness of such crystals and their chemical reactivity with most sensors.
The best known method for avoiding sensors and measuring temperature non-invasively during the growth of crystals is radiation pyrometry. Radiation pyrometry is the measurement of the radiation which all objects emit continually in accordance with known physical laws of radiation. The characteristics of the radiation depend, in part, on the temperature of the object. Optical or infrared radiation pyrometries are useful over a wide temperature range, and by selecting a spectral range in which the object is opaque, the measurements can be said to pertain to the surface temperature. However, radiation pyrometry is not appropriate for monitoring the growth of .alpha.-HgI.sub.2 crystals. There is enough radiance at the growth temperatures of the .alpha.-HgI.sub.2 crystals to be measured by modern infrared sensing instruments, although the emissivity is relatively low in the infrared spectrum where the Planck radiation peak is located. However, because the crystal is transparent in this spectral range, the detected radiance represents the underlying bulk, as well as the surface, temperatures. Since the crystal during growth has a temperature gradient between its surface and interior, the analysis of its radiance will yield a depth-to-surface average temperature, rather than being specific to its surface. In a different situation, such as during annealing (where the crystal is isothermal), infrared radiation pyrometery could be useful, if the usual precautions are taken to eliminate background radiance transmitted through and reflected from the crystal. If the transparency problem is avoided by measuring the radiance at wavelengths below approximately 580 nm, at which frequency the .alpha.-HgI.sub.2 crystal is opaque, the intensity is immeasurably small at the temperatures of interest.
Clearly, it would be advantageous to provide a method which could accurately measure the surface temperature of growing .alpha.-HgI.sub.2 crystals by non-invasive methods. However, to the inventors' knowledge, no prior art method has successfully accurately measured the surface temperature of growing crystals, such as the .alpha.-HgI.sub.2 crystals, by non-invasive methods. All prior art successful methods for measuring the surface of such growing crystals have either been of an invasive type or else have measured temperatures which were not truly accurate surface temperatures when the crystals had a surface to bulk temperature gradient.